Forget Cheetah Blades. This Prosthetic Socket Is a Real Breakthrough

San Francisco Prosthetic Orthotic Service, located just around the corner from the Painted Ladies, is a fairly standard prosthetics shop. There’s a receptionist or two in front, with some small rooms behind them where people get fit with prosthetics. Past the rooms, beyond an unmarked door, there’s a workshop where prosthetic parts are made. Here, tools hang on the walls above ovens and lathes, and everything seems to be covered in a fine coating of white plaster dust.

But down a narrow flight of stairs, in the basement, there’s a different sort of prosthetics operation at work. Instead of saws, it has 3D scanners. This is the de facto lab for LIM Innovations, a small start-up whose work stands to dramatically improve the lives of countless amputees. LIM’s creation, the Infinite Socket, is a complete rethinking of a crucial prosthetic component, bringing modern technology and thoughtful design to bear on a long-ignored pain point. Where traditional sockets are good for maybe an hour of comfortable walking, an early tester of LIM’s said he walked eight miles around hilly San Francisco on his first day with the prototype.

The Problem With Sockets

It’s not unusual to hear about the latest advances in prosthetic limbs. Think carbon fiber cheetah blades and electronic knees controlled by microprocessors. Less glamorous, but just as important, are prosthetic sockets—the pieces connect those sophisticated prosthetics to peoples’ residual limbs. The prosthetic limb may look cool, but the socket is the real point of contact. It’s the interface where the hardware meets the human.

When Garrett Hurley, a prosthetist, and Andrew Pedtke, an orthopedic surgeon, met at University of California San Francisco in 2011, both were familiar with how sockets were traditionally made. It’s an enormously tedious process. You start by making a plaster mold of a person’s residual limb. Then someone in a shop works the mold by hand, trying to make sure it takes pressure in the right places. At this point, a prosthetist might make a trial socket out of glass, so they can see how the thing actually fits with the leg. You wait some more, and then you end up with a socket.

This process takes around a month—longer for complex cases—and a handful of trips to a prosthetist. But the wait isn’t even necessarily the worst part. Hurley and Pedtke also knew that after all that, there’s still a decent chance the damn thing won’t fit.

Sockets have a rigid, hard-shell design. Human limbs, however, can shrink and swell by up to ten percent in volume. “One-off fabrication is great if you want to create a snapshot in time. But the limb is dynamic. The limb changes,” Hurley says. This leaves it up to patients to figure out how to re-achieve a decent fit. It isn’t unusual for someone to layer a half-dozen socks on their stump to get things snug inside a socket. Even then, blisters and sores are common.

It didn’t take much for a prosthetist like Hurley to see the problems with this system. “I’ve been making legs for people for 12 years now,” he says. “At some point it just struck me: There’s gotta be a better way.”

A Modular Design With an Adjustable Fit

Over a series of international trips, with time split between hospital work and surfing, Hurley and Pedtke started thinking about how they might improve the lowly socket. They knew that new technologies, like 3D scanning, could help with the lengthy fitting process. But the bigger problem was the design of the socket itself. As long as they were rigid all the way around—as long as they were just a snapshot in time—there would always be trouble with the fit.

The answer, they determined, was modularity. Pedtke likes to explain the approach by comparing it to shoes. Sockets have traditionally been like wooden clogs—made by hand, out of a single material, and ultimately rather unforgiving to wear. The folks at LIM came to think of their socket like a sneaker, with different parts made from different materials, each designed for a specific task.

An early tester putting on the Infinite Socket. WIRED Design

Jesse Williams, a materials science PhD, led the engineering effort. Smart Design, the San Francisco firm behind the hugely popular kitchenware brand OXO, was brought on to refine the design. What they ended up with differs from traditional sockets in a number of very important ways.

It starts with the fitting. Instead of the messy plaster cast, LIM works from a 3D scan of an amputee’s leg. Then they use computer software to model four struts, which serve as the rigid frame of the socket. The struts are made from carbon fiber strips, which are heated and smashed into place with a modified CNC machine. By relying on an acrylic-based variety of carbon fiber instead of the standard epoxy-based version, the struts can be heated up and reshaped later, if tweaks are necessary.

The struts are held together at the top by something LIM refers to as the soft-goods interface. This is the most radical aspect of the Infinite Socket and the part that really stands to improve the day-to-day life of the person wearing it. The top of a traditional socket has a hard, fixed circumference. LIM’s has a thick foam cuff. The crucial part isn’t the padding. It’s the adjustability.

The socket’s tension can be increased or decreased depending on activity. LIM Innovations

Using a simple ladder lock buckle—a detail borrowed from snowboard boot bindings—a person wearing the Infinite Socket can ratchet up tension or release it just by sticking their hand in their pocket. This adjustability not only solves the limb-swelling problem that plagued previous rigid socket designs. It also lets people adjust tension depending on what they’re doing. They might want it super tight, for instance, if they’re playing tennis, and much looser when they plop down on the couch afterward.

But the socket also has to help bear the weight of the person walking on it. For this, LIM and Smart Design developed an ischial seat, a tiny ledge that sits just above the soft part of the socket, underneath the wearer’s butt. In old sockets, this function was served by the rigid brim at the top of the cup. By separating it out into its own component, LIM was able to rapidly prototype the component as they zeroed-in on the ideal size and shape.

The Biggest Advantage: Empowering People

Part of the idea with the Infinite Socket’s design was simply speeding up the time it takes to make a socket. By selectively choosing which parts of the socket are standard and which need to be customized, LIM can generate those parts faster. And because many of these parts can be adjusted in various ways, it’s easy for a prosthetist to come in at the end dial in the perfect fit.

Tom Dair, the founder of Smart Design, likens it a bicycle. It might come in a box, but a bike mechanic can adjust the seat and the pedals and the handlebars to get things just right. “The prosthetist will be able to take this modular system and be able to tailor it to the individual to meet their needs,” he says. LIM’s currently looking for partners in the health care industry, but they envision a future in which someone could use a smartphone to scan their own limb and get a socket back for this final fitting within 24 hours.

Still, the real breakthrough of the Infinite Socket is giving amputees some control over this very important part of their everyday lives—in a way that doesn’t involve strapping on seven or eight socks to pad things out. LIM’s adjustable interface gives socket wearers a crucial sort of agency for the first time. Beyond physical comfort, there’s a psychological benefit that comes with that too. “When you invite a home glucose meter, you’re not telling the person you’re going to cure them of diabetes,” Pedtke says. “You’re putting the disease into their hands so they can manage it better. That’s what we’re doing. And that’s not available with old sockets.”